V. TEMPERATURE DETERMINATIONS 135 



tions and, therefore, may be used advantageously as the expansile 

 liquid in a sensitive thermometer below 60°C. In this range an 

 alcohol thermometer having the same sensitivity as a mercury 

 thermometer can have a bulb size one-tenth that of a mercury 

 thermometer of the same size bore. Furthermore, an alcohol 

 thermometer has less tendency to cool the medium being tested 

 than a mercury thermometer and may be used at lower temperatures 

 since alcohol does not solidify until it is cooled to — 130°C. How- 

 ever, alcohol boils at 78°C. and cannot be used near or above this 

 temperature except under pressure. The stem of the alcohol ther- 

 mometer must be kept at the same of at a higher temperature than^ 

 the bulb because of the possibility of condensation on the inner wall. 



Mercury-in-Glass Thermometers. Mercury-in-glass thermome- 

 ters are the most commonly used temperature-measuring instru- 

 ments in the laboratory. Mercury, when allowed to expand in only 

 one dimension, expands and contracts linearly with temperature 

 changes between its solidification and vaporization points ( — 39 to 

 357°C.). In general, mercury-in-glass thermometers are inexpen- 

 sive, easily calibrated, and can be obtained in many sizes and ranges 

 for the degree of sensitivity required. 



Mercury is confined to a reservoir (bulb) attached to a length of 

 glass tubing (stem), which is sealed in the upper end after evacuation. 

 For use at temperatures between 350 and 500°C. the thermometer is 

 made of special glass with a high melting point and is filled with nitro- 

 gen under pressure to raise the boiling point of mercury. Either 

 enclosed (Einschuss) or the more popular etched-stem scales are ob- 

 tainable. Etched-stem types tend to lose the coloring matter filling 

 the etched lines and become difficult to read, but they are less ex- 

 pensive and less bulky. Enclosed-scale types have the temperature 

 divisions marked on a white background placed behind the stem, both 

 being enclosed in a glass envelope. The large, well protected scale 

 of this type is easy to read but may become loose and slip away from 

 the capillary tubing of the thermometer. Enclosed-stem thermome- 

 ters need not be discarded if the glass envelope becomes broken since 

 the latter may be replaced. 



It is most important to distinguish between accuracy and sensi- 

 tivity in a thermometer. Sensitivity, the ability of the thermometer 

 to respond to temperature changes, depends upon the ratio of the 

 volume of the bulb to the cross-sectional area of the bore of the tube. 

 To increase the sensitivity for small increments of temperature, the 



